The present invention relates to a display apparatus, and more particularly, to a display apparatus which comprises a direct-heating type cathode of an impregnated structure for displaying pictures virtually instantaneously after the apparatus is turned on.
Generally, display apparatuses can be formed as monitors, televisions, liquid crystal displays, etc. Among these options, the monitor and the television include a cathode ray tube (CRT) and a CRT driving circuit, for displaying pictures on a screen.
Generally, a cathode for use in the CRT emits thermions by heat energy. Such cathodes are largely classified into two groups, namely, a heat-emissive type cathode, which is an indirect-heating type cathode, and a direct-heating type cathode. As shown in FIG. 1, the indirect-heating type cathode has a structure in which the heater is separated from the cathode. A display apparatus adopting such a heat-emissive type cathode is shown in FIG.2. Here, a color television has been adopted as the display apparatus.
Referring to FIGS. 1 and 2, if the power switch of the color television is turned on, power is applied to a heater 4. Then, heater 4, formed of tungsten material on which an insulation material 41 is coated, generates heat after a few seconds. In this case, if leakage current flows between a base metal 1 forming the cathode and heater 4, heater 4 can be destroyed, or a video signal voltage and a cut-off voltage applied to cathode 1 may vary. Accordingly, insulation material 41 is used for avoiding the leakage of current between heater 4 and cathode 1.
In the case of a color CRT, a flyback pulse voltage or a direct voltage is used as the power applied in order to heat up heater 4. The flyback pulse has a voltage value of e.g., 6.3 Vrms and a current value of 600-700 mArms, which is produced from the secondary winding of a flyback transformer (not shown). Here, the power consumption is about 4.4 watts.
Heat generated by heater 4 is transmitted to a cathode sleeve 2, a base metal 1 and a holder 3 in sequence by radiation and conduction, so that an electron-emitting substance 11, coated on base metal 1, is heated up until the electron-emitting substance 11 reaches a normal operating temperature, that is, a proper temperature for emitting thermions.
Thus, since the cathode of the cathode-ray tube used in the conventional display apparatus is a heat-emissive type cathode separated from the heater, heater 4, as a heat source, is spaced at a predetermined distance from a carbonate, as the electron-emitting substance 11. Therefore, the electron-emitting substance 11 is heated up gradually, so that it takes a predetermined time (about ten seconds) until the electron-emitting substance reaches a normal operating state. Consequently, cathode-ray tubes utilizing the heat-emissive type cathode have a drawback in that the time required to display pictures becomes overly long.
Also, the respective initial times for emitting the thermions by plural cathodes may fail to coincide, e.g., in a situation where three cathodes respectively corresponding to red (R), green (G) and blue (B) signals are provided. Also, another drawback may arise in that the picture's white balance may be distorted (for instance, if the cathode corresponding to the R signal emits thermions faster than those for the G and B signals, the whole image shows a red tint), such that the initial quality of the image is lowered. Further, since it takes a relatively long time for the overall white balance to adjust, problems arise in efficiency of the manufacturing process.
Also, since a carbonate is usually used as the electron-emitting substance for heat-emissive type cathodes, when a high current is applied thereto, joule heat is generated therein, which is undesirable. Therefore, heat-emissive type cathodes cannot be used in high-definition televisions (HDTV), which should be operated at a high current density.
To solve the above problems, a direct-heating type cathode has been developed wherein the thermions are emitted from the electron-emitting substance in a short time, so that the initial time required for displaying images in the CRT can be reduced.
FIG.3 is a detailed view of a cathode portion where a filament 12 is secured to a cathode matrix 11 having an impregnated structure. Such a cathode portion is useful in direct-heating type cathodes. A detailed explanation of the portion shown in FIG.3 is disclosed in Korean patent application No.91-9461 for the invention entitled "A Direct-heating Type Cathode of an Electron Gun for a Cathode Ray Tube and the Manufacturing Method Thereof." by Samsung Display Devices Co., Ltd.
To summarize the contents of the above Korean patent application, a direct-heating type cathode mounted in the electron gun for use in the CRT to emit thermions is formed of a cathode matrix 11, into which the electron-emitting substance, e.g., cesium, is impregnated. A heater 12 made of an alloy of molybdenum (Mo) and rhenium (Re) is welded to the cathode matrix 11. After power is applied thereto, heat generated from heater 12 is transmitted to cathode matrix 11 directly. Accordingly, the thermal efficiency in which heat is transmitted from heater 12 to cathode matrix 11 can be enhanced. This, in turn allows the portion to be adapted to HDTV systems requiring electron-emitting characteristics of high current density, and reduces the time needed for displaying pictures in the CRT.
Since monochrome CRTs have only one cathode, video signals can be overlapped in a G1 grid (FIG.2) of the CRT. However, when a CRT having one or more cathodes, especially a color CRT, adopts a direct-heating type cathode of an impregnated structure, where the cathode electrically contacts the heater, as shown in FIG.3, only a few methods are available in which both the video signal overlapped with the cut-off voltage and the power for driving the heater can be applied simultaneously.